It is desirable or necessary in a number of apparatus and processes to sense the rate of change of changing rotational speeds of a rotatable element. One example of such a need is found in connection with the use of brake modulators for varying the braking effect exerted on a rotating member such as an automotive vehicle wheel. A variety of approaches to sensing rates of change of changing rotational speeds and/or vehicle wheel slip have been proposed heretofore, as will be recognized by persons familiar with prior patents relating to the field of this invention.
In connection with the use and development of such sensors and braking arrangements incorporating such sensors, it has become common to define the operating conditions for a system in part by a so-called "skid number", referring to the multiple by a hundred of a coefficient of friction between a vehicle wheel and a road surface over which a vehicle moves. Prior sensors, particularly those employing a frictional coupling and a physical contact or microswitch electrical signalling arrangement, have heretofore achieved success for skid numbers of approximately 80 or higher where much vehicle operation occurs. Nevertheless, as work with such sensors has continued, it has been found desirable to provide a sensor with long operating life, which is easily maintained, and which is adaptable to a wide range of skid numbers and vehicle operating conditions.